Question

I keep getting this error in my user class:

User is not abstract and does not override abstract method compareTo(User) in Comparable

I’m not sure what it means or what to do to make it work. I did look on other similar posts but don’t really understand the responses since they are most of the time specific to someone elses code. I can provide more information about what I’m trying to do if required.

public class User implements Comparable < User > {

    private String firstName;
    private String surname;
    private int booksOut;

    User(String inFirstName, String inSurname, int inBooksOut){
        this.firstName = inFirstName;
        this.surname = inSurname;
        this.booksOut = inBooksOut;
    }

    //...

}

What i am essentially doing I am creating SortedLinkedList class from LinkedList and I have add method in there which uses compareTo, to order the data in ascending order as they are added.
So when I call

 User user1= new User("Dan", "Hill", 0); 
SortedLinkedList LibraryUser = new SortedLinkedList(); 
LibraryUser.add(user1);  

It creates new object which is saved in the sorted linked list.

Answer 1

When you implement Comparable, java looks for the implementation of compareTo

For example:

(Please note that I made the variables public for simplicity)

public class User implements Comparable < User > {

    public String firstName;
    public String surname;
    public int booksOut;

    User(String inFirstName, String inSurname, int inBooksOut){
        this.firstName = inFirstName;
        this.surname = inSurname;
        this.booksOut = inBooksOut;
    }

    public int compareTo(User cmp) {
        int fnCmp = firstName.compareTo(cmp.firstName);
        if (fnCmp != 0) return(fnCmp);
        int snCmp = surname.compareTo(cmp.surname);
        if (snCmp != 0) return(snCmp);
        return(cmp.booksOut-booksOut);
    }   
}